In the thermal analysis, properties of a material are examined by means of a sample from the respective material as a function of the temperature. A correct thermometry, i.e. a correct measurement of the temperature of the sample, plays an important role hereby.
Temperature sensors in the form of thermocouples or electrical resistance thermometers, e.g., which can be arranged in thermal contact to the sample, e.g., in order to measure the temperature of a sample during the examination thereof, are known from the prior art of devices for the thermal analysis of samples.
This measurement, however, may be more or less error prone, for instance because such a temperature sensor does not measure the temperature in the interior of the sample, but, e.g. at an edge of the sample. This measuring error is even more significant for temperature sensors, which are frequently used and which cannot be arranged in direct contact to the sample, but which are instead arranged adjacent to the sample, thus spatially separated therefrom, inside a sample chamber of the device.
This problem can be reduced significantly by means of a suitable “calibration” of the used temperature sensor or the temperature measuring system formed therewith, respectively. Calibration means that one or a plurality of correction parameters are determined and stored, in order to be able to consider them accordingly in response to subsequent thermal analyses.
For such a calibration, it can be provided, e.g., to measure the temperature of one or a plurality of samples, which are each to be melted at known temperatures, in the respective device for the thermal analysis during the course of a temperature program (predetermined temporal change of the sample temperature), in order to then use a comparison of the melting temperatures measured by means of the temperature measuring system with the melting temperatures, which are known, e.g. from the literature, to calibrate the temperature measuring system.
A sample comprising a known Curie temperature, e.g., can also be used as an alternative to the use of such a melting standard for the calibration.
Devices for the thermal analysis, in the case of which such a calibration of the temperature measuring system can be carried out, e.g. in the above-described manner, are known from the prior art in a variety of designs. One example for this is the device LFA 467 HT HyperFlash” by Netzsch-Gerätebau GmbH, Selb, Germany. This known device comprises:                a sample chamber comprising a plurality of sample holders, which are in each case embodied to hold a sample,        for each of the plurality of sample holders, a controllable temperature control system (here: “mini tube furnace”) assigned to said sample holder, for controlling the temperature of the sample held by this sample holder,        a temperature measuring system (here: comprising a plurality of thermocouples in each case assigned to one of the sample holders) for measuring the temperature of the samples,        a photothermal measuring system for irradiating a first side of the samples comprising an electromagnetic excitation pulse and for capturing a thermal radiation emitted by a second side of the samples located opposite the first side as a result of the excitation pulse,        a control and evaluating system for controlling the temperature control systems and the photothermal measuring system and for recording measuring data, which represent at least one physical property of the samples (here: temperature conductivity and/or specific thermal capacity), which differs from the temperature of the samples, as a function of the temperature of the samples.        
A disadvantage of the known generic calibrating methods is, for example, the effort associated therewith and the limitation to very specific “calibrating temperatures” (in the case of which a melting or another easily detectable phase conversion, respectively, takes place).
When the calibration is to take place across a relatively large temperature range, this is made more difficult in that calibration measurements must in each case be carried out repeatedly with different samples, which serve as standard, for each individual sample holder (each “sample or measuring position”, respectively). The different phase conversion temperatures of these samples then provide a corresponding plurality of “support locations” (calibrating temperatures) relating to the respective temperature range.
In addition, possible chemical material reactions must always be observed as well, which is why for example a cobalt standard sample (comprising a known Curie temperature of approx. 1115° C.) cannot be used in a siliceous sample holder (e.g. of SiC), because the cobalt reacts with silicon at higher temperatures.
Based on a calibrating method of the above-discussed type, it is an object of the invention at hand to overcome the disadvantages thereof and to specify an alternative calibrating method.
The method according to the invention for calibrating a device for the thermal analysis of the above-mentioned type comprises the following steps:                carrying out photothermal measurements by means of the photothermal measuring system        on a certain sample, which is consecutively held in the plurality of sample holders for this purpose and which is in each case subjected to a photothermal measurement, or        on a plurality of similar samples, which are in each case held in one of the plurality of sample holders for this purpose and which are in each case subjected to a photothermal measurement,        wherein in the case of the photothermal measurements, a first side of the respective sample is in each case irradiated with an electromagnetic excitation pulse and a thermal radiation emitted by a second side of this sample located opposite the first side is captured as a result of the excitation pulse,        comparing results of the photothermal measurements for the plurality of sample holders,        in each case determining at least one correction parameter for each sample holder based on a result of the comparison,        calibrating the temperature measuring system and/or the temperature control systems based on the determined correction parameters.        
The basic idea of the invention is to use the photothermal measuring system, which is already present in a respective device, for initially carrying out a photothermal measurement (of the type provided according to the invention) for each of the plurality of sample holders or synonymously the plurality of “sample positions”, respectively, wherein it would “normally” be expected on the basis of the use of a certain sample or a plurality of similar samples that the measurements provide identical results, in order to then use differences of the results, which do in fact appear in practice, to determine correction parameters for the individual sample holders or sample positions, respectively, so as to finally calibrate the temperature measuring system and/or the temperature control systems of the device on the basis of the determined correction parameters.
In a particularly advantageous embodiment of the calibrating method, the above-mentioned method steps are carried out, after a calibration of the temperature measuring system (and/or of the temperature control system), which is as accurate as possible, with regard to the temperature measurement (or temperature control, respectively) had already been carried out beforehand for (at least) one sample holder or (at least) one sample position, respectively, in any suitable manner for this sample position (e.g. by means of a melting standard or the like). The above-mentioned method steps then provide for a calibration of the temperature measuring system, which can be carried out very easily by means of the correction parameters, with regard to the temperature measurement of the remaining sample positions.